The overall objective when producing organic saturated aliphatic monocarboxylic acids containing from 6 to 9 carbon atoms by oxidation from their corresponding aldehydes, is to obtain the highest yields and product efficiencies at the highest conversion levels. The use of mixtures of copper and manganese compounds soluble in the acid product as catalysts helps achieve this objective, and is disclosed in copending U.S. patent application Ser. No. 345,890, filed on Feb. 4, 1982 in the names of Hobbs, Jr. and Thigpen and assigned to Celanese Corporation. However, copper has a tendency to plate out in the distillation apparatus used for acid product recovery, resulting in undesirable mechanical problems such as erosion of reboilers and pump impellers and rapid pump seal failures.
A method of overcoming this copper plating problem, involving adding oxalic acid to the acid product to precipitate copper and manganese as the oxalates, then filtering the oxalates from the acid product prior to the acid distillation step, is described in U.S. Pat. No. 4,289,708, issued Sept. 15, 1981 to Scott et al and assigned to Celanese Corporation. Copper and manganese can also be separated from the acid product, again as the oxalates, by adding aqueous oxalic acid to precipitate manganese and copper oxalates into the aqueous phase, which is then separated from the acid product by decantation of the acid. The acid can then be further purified by distillation. This latter process is described in U.S. Pat. No. 4,246,185, issued Jan. 12, 1981 to Wood, Jr. and assigned to Celanese Corporation.
The foregoing separation processes result in the production of substantial amounts of cupric and manganous oxalates. It is the purpose of this invention to convert these oxalates to alkanoates useful as catalysts for recycle in the oxidation of C.sub.6 -C.sub.9 saturated aliphatic aldehydes to the corresponding monocarboxylic acid.
U.S. Pat. No. 3,133,942, issued May 19, 1964 to Hahl and assigned to Badische Anilin- & Soda-Fabrik Aktiengesellschaft, describes a technique for the production of metal salts, including copper salts of organic acids, including saturated aliphatic monocarbolic acids having 1 to 8 carbon atoms, oleic acid and benzoic acid, which involves reacting the metal with the acid in the presence of free oxygen. The reactions of n-butyric, n-valeric and n-caprylic acids with copper powder in the presence of oxygen to produce copper (II) butyrate, copper (II) valerate and copper (II) caprylate, respectively, are specifically disclosed. Hahl makes no mention, however, of conducting such reactions in the presence of manganese compounds such as manganese alkanoate and the products obtained by the thermal decomposition of manganous oxalate and cuprous oxalate in the presence of a C.sub.6 -C.sub.9 saturated aliphatic monocarboxylic acid. In particular, Hahl has no teaching of the conversion of the oxalates suspended in an alkanoic acid to form metallic copper and manganese alkanoate directly. Finally, Hahl's teaching does not include the treatment of manganese in any form whatever to obtain a manganese alkanoate since the normal potential of manganese is -1.029 volt and thus does not fall within the range of the normal potential of -0.80 and +0.5 volt described by Hahl.